Pitfalls in determining the dead time of nuclear well-logging probes

The accurate determination of the dead time of nuclear well-logging probes is important for quantitative interpretation of uranium ore grade from natural gamma-ray logs, of density from gamma-gamma logs, and of hydrogen content (water or hydrocarbons) from neutron logs. Several empirical techniques for determining dead time are in common use, but only one of them, the two-source method, is considered reliable. Two reliable formulas are presented and referenced. One represents an accurate approximation, the other an exact two-step calculation. Background radiation should be considered when making two-source measurements, and two different techniques are recommended for applying background corrections; one for neutron and the other for gamma-ray measurements. Examples of the two-source method for determining the dead time of neutron and gamma-ray measurement probes are presented. 12 refs